1. Field of the Invention
The invention relates in general to a charge-coupled device (CCD) sensing apparatus, and more particularly to a CCD sensing apparatus with dual photo sensor sets, which help retain image quality.
2. Description of the Related Art
Generally, a charge-coupled device (CCD) sensing apparatus in a carriage is applied for transferring a photo signal to an electrical signal in a scanner. A conventional CCD sensing apparatus includes at least a photo sensor set and a CCD shift register. The photo sensor set includes a number of photo sensors. The photo sensors may be photo diodes. After the photo sensors are exposed to light for a predetermined period of time, the photo sensors generate charge signals, and then the charge signals are sent to the CCD shift register. The CCD shift register can be a two phase CCD shift register. The charge signals shifted out from the CCD shift register are stored serially in a capacitor so as to generate the analog output signals. In the next stage, a circuit processes the output signals, and the digital scanned image data are then generated.
Conventionally, a stagger type CCD sensing apparatus is used in order to enhance resolution. In FIG. 1, a conventional stagger type CCD sensing apparatus is shown. The CCD sensing apparatus 102 includes a photo sensor set 106A, a photo sensor set 106B, a CCD shift register 110A, and a CCD shift register 110B. The photo sensor sets 106A and 106B are staggered. The CCD shift registers 110A and 110B receive the charge signals transmitted from the photo sensor sets 106A and 106B, respectively. Then, charge signals output by CCD shift registers 110A and 110B are stored serially in capacitors C1 and C2, respectively, to generate the analog output signals Out1 and Out2. After the output signals Out1 and Out2 are processed by a circuit in the next stage and are combined together, the scanned image data with double resolution of that of the photo sensor sets 106A or 106B are then generated.
However, minor difference in the process of producing the two CCD shift registers usually occurs. As a result, the properties of the two CCD shift registers may not be exactly the same. For example, differences in charge signal output rate and the decay rate of the charge signals could exist when the charge signals are output. Thus, the scanning quality differences may be caused by using the two CCD shift registers. Consequently, this inconsistency deteriorates image quality when the next stage circuit combines the output signals Out1 and Out2. To overcome this problem, many image processing procedures were proposed. However, most of them are complex and time-consuming.